It is easier to learn a language being immersed in it. I think it's
incredibly more effortless after you have had some basic grammar, structure, and vocabulary flow down before you fully immerse yourself.
If you know the basic grammar structure and can understand most of the more common-used words, it will be a hell of a lot easier to pick up and start speaking sooner.
If you know even a minimal amount, just enough to express [poorly] most general things you'd need to say, you'll probably gain A LOT more from being there for a year.
Suppose you can understand what everyone is saying and reply or give feedback to what is displayed. In that case, this will allow you to expand your language skill much further and faster than just hearing jibberish and being unable to say anything. Not knowing anything will make it daunting and frustrating.
I've been around a lot of exchange students the past few years. Whenever I talk to the outbound students and ask how much of the language [of the country they are going to] they speak, they usually tell me,
"None really, I know I'll learn it once I'm there just naturally" This is true for the program we are in, because you go to school and live with a family, so you'll hear the language ALL the time. I think that's a stupid approach, though. Imagine how much more you will learn if you start learning it before going to Japan.
I have two friends that went to Japan. One of them just left, and I haven't heard from her since she arrived. She knew a decent bit of Japanese before she left and had the drive to learn it. I think her experience will be much more enjoyable. The other person who went to Japan said stuff like,
"We went to an orientation, but I felt it was such a waste of time. We could have been outside having fun, but instead, we just sat inside and listened to people talk for hours and hours. They were talking in Japanese too, so it was just a complete waste of time. I just zoned out and stuff" Since he made no effort to learn any before he left, he lost a lot of opportunities to learn fluent Japanese in a year. He could have used those "Talking for hours and hours" as an opportunity to learn new words and get a better feel for the language.
It will make it a hell of a lot easier on you if you take some classes beforehand. The effort required to learn before you go will be nothing compared to the stress on your brain if you go without adequate preparation ahead.
I can understand why you don't want to learn Japanese, as it's so hard and you could learn so many other languages in the time it would take. I agree with you there, I think. I probably wouldn't want to live there. I still have a lot of German languages I want to learn. If you will live there for a year, though, do yourself a favor and start now.